Just a couple of things to add on to what Coach Charles said.
First my background. I have been teaching fitness since 1998 and have over my career been a personal trainer, group fitness instructor, Pilates instructor, and adjuct instructor at Bastyr University. I currently own my own Pilates studio in Redmond, WA. I have my BS in exercise science and wellness, am NSCA CSCS cerified, Stott Pilates Certified, and ACE Group Fitness Instructor certified. I am currently about 1/2 way through completing my master's degree in kinesiology. I love Disney and just started checking out the WISH boards recently because I am doing the 2013 Disney Marathon. I'm normally more of a lurker than a poster.
While I would agree with Coach Charles about the usefullness of max HR formula's I respectfully disagree about it being a misapplication of one study. Many, many studies have been done and it is actually useful on a population level but the problem is that the 220-age reccomendation has a standard deviation of over 10 beats per minute (I forget exactly what it was but it was shockingly high). Since statistics tells us that 95% of a population will fall within 2 standard deviations, the 220-age +/- 20ish beats is actually where it falls which is less than helpful for an individual.
That said if people follow the guidelines as given they will improve in fitness and if the op is the sort of person who feels more secure to have a device telling her she is doing ok that is perfectly fine. Op for more info on heart rate monitors start here
http://www.acsm.org/docs/brochures/selecting-and-effectively-using-a-heart-rate-monitor.pdf.
Coach Charles's suggestion for finding AT was interesting and would be effective and useful but does require one to enter the vigorous category and it would be remiss to not mention proper health screening. The ACSM's guidelines of who should recieve medical screening and/or fitness testing before exercsie can be found here
http://www.acefitness.org/pdfs/ACSM-CAD-Risk-Factor-Chart.pdf
It is shockingly easy to be in the moderate risk category at which point it is usually recommended to stick with low-moderate intensity exercise unless one has medical clearance. If you are in the low risk category or get clearance his protocol looks good.
Good luck with restarting your fitness journey. I would like to restate my belief that in the beginning the most important thing is to start creating good habits. The most perfect program become useless if one stops doing it after a couple of weeks or develops an overuse injury and is forced to stop.